1. Field of the Invention
This invention relates to a length measuring apparatus for detecting a distance up to an object to be measured and a displacement of the object, and more particularly, to a length measuring apparatus for accurately measuring a displacement and a position of the object utilizing coherent light, such as laser light or the like. The apparatus is suitably used, for example, for the control of a stage of an exposing apparatus for semiconductor devices.
2. Description of the Prior Art
Heretofore, as a length measuring apparatus for the control of a stage of an exposing apparatus for semiconductor devices, a so-called two-wavelength-laser interferometer has been known which measures a distance of movement by utilizing a Doppler shift in optical frequency caused by the movement of a mirror mounted on the stage. This approach is termed a first conventional example.
FIG. 1 shows a diagram of the configuration of such an apparatus for explaining the principle of measurement in the first conventional example.
In FIG. 1, there are shown a two-frequency Zeeman laser oscillator 1, serving as a light source, a beam splitter 2, an interferometer unit 3 consisting of a polarizing beam splitter and a corner cube, a .lambda./4 plate 4, a plane reflecting mirror 5, polarizers 6a and 6b, photodetectors 7a and 7b, pulse converters 8a and 8b for converting sinusoidal signals into pulse trains, an up/down counter 9 for performing addition/subtraction of pulses, and a stage 10.
In the above-described configuration, two light beams P and Q emitted from the two-frequency Zeeman laser oscillator 1 are electromagnetic waves having frequencies of f.sub.1 and f.sub.2, respectively, and are linearly polarized light beams orthogonal to each other. Each of the light beams P and Q is divided into two beams by the beam splitter 2. Deflected light beams interfere with each other by the function of the polarizer 6a, and the resultant light beam is detected as a beat signal having a frequency f.sub.1 -f.sub.2 by the photodetector 7a. This signal is made a reference signal.
The straight-going beams enter the interferometer unit 3, and are divided into light beams P and Q by the function of the polarizing beam splitter. The light beam Q exits after passing through only the interior of the interferometer unit 3, and the light beam P exits after being reflected twice by the plane reflecting mirror 5 mounted on the stage 10. The light beams P and Q interfere with each other by the function of the polarizer 6b, and the resultant light beam is also detected as a beat signal having a frequency f.sub.1 -f.sub.2 by the photodetector 7b. The beat signals detected by the photodetectors 7a and 7b are converted into pulse trains by the pulse converters 8a and 8b, respectively, and the difference between the numbers of the pulses is counted by the up/down counter 9.
In this state, if the stage 10 moves at a speed v in the direction of the optical axis, the light beam P reflected by the mirror 5 on the stage 10 is subjected to a Doppler shift per one reflection of EQU .DELTA.f=2v/C.multidot.f.sub.1 ( 1),
where C is the velocity of light. Since the light beam P is reflected twice in the configuration in FIG. 1, the light beam P subjected to a Doppler shift of 2.DELTA.f is incident upon the photodetector 7b. Hence, the frequency of the signal detected by the photodetector 7b changes to f.sub.1 -f.sub.2 .+-.2.DELTA.f. To the contrary, the signal detected by the photodetector 7a remains to be f.sub.1 -f.sub.2. As a result, the output from the up/down counter 9 becomes .+-.2.DELTA.f. The amount of movement of the stage 10 is obtained by multiplying this output value by the wavelength of the light beam P. Thus, in the conventional apparatus, the amount of displacement of the stage is incrementally obtained.
On the other hand, as disclosed in Japanese Patent Public Disclosure (Kokai) Nos. 62-135703 (1987) and 62-204103 (1987), a method has also been devised in which absolute position and displacement are measured using light sources having different wavelengths. This method is termed a second conventional example. In this method, a phase difference .phi..sub.1 in interference fringes obtained by a wavelength .lambda..sub.1 has the following relationship: EQU l=(2.pi.N+.phi..sub.1).lambda..sub.1 /2.pi. (2),
where l is the optical path difference of an interferometer, and N is a natural number. Hence, a range that an unknown natural number N may have is gradually restricted by measuring phase differences .phi..sub.2, .phi..sub.3 ---for various wavelegths .lambda..sub.2, .lambda..sub.3 ---, and an absolute position l is obtained by finally uniquely determining the natural number N.
However, the first conventional example has a disadvantage in that, since the distance of movement is measured by obtaining the integral of the difference between the reference pulses and measurement, it becomes impossible to perform measurement if the laser light is shut off even for a moment. The example also has a disadvantage in that, since only the amount of displacement from a point which has been reset can be measured, it is necessary to separately provide an origin sensor for, for example, the control of a stage, and hence the system of the stage becomes complicated.
On the other hand, although an absolute measurement can be performed in the second conventional example, and hence the disadvantages in the first conventional example are eliminated, the second conventional example has a problem in that it is difficult to perform real-time monitoring of the position of a stage moving at high speed, since a complicated method of measurement is needed.